Android :: How To Protect Private Information From Others?

I've integrated lots of private information like accounting, messaging, emails, tasks and so on, into desktop widgets and everywhere around the phone. Every so often, someone wants to take a look at the phone and play around. Reading some important stuff is one thing, but people tend to mess the stuff around as well. Have you found a program or a way to protect from that? (other than denying to the people :> ) I guess some kind of shell program that you turn over when someone wants to demo the phone would be a nice solution.